After a bright start the teenager was outgunned by Ekaterina Alexandrova 6-4 6-3 in 75 minutes on a sweltering Monday in the US Midwest.
Opening play on a sparsely-attended Court 3, the 19-year-old gained an early break in the last-32 tie, and was 2-0 and 4-2 up. But the 12th-seeded Russian stepped up the power to take the set with two breaks.
The second stanza was more closely contested until Alexandrova won the sixth game to take a 4-2 lead and served out, concluding with an ace.
Joint had forced five break points but took only one while Alexandrova won all three she created.
Compatriots Daria Kasatkina, Ajla Tomljanovic and Kimberly Birrell all lost their second-round matches on Sunday. But being ranked within the top 100 all should be confirmed in the main draw for the US Open which begins on August 24.
However, Joint's chances of joining Kasatkina among the seeds in New York, which might have been achieved with a deep run in Cincinnati, have gone.
Joint, who was born and grew up in the neighbouring state of Michigan, is currently entered for another Ohio event next week, the WTA 250 in Cleveland.
She will hope to pick up the form that enabled her to defeat 18th-seeded Brazilian Beatriz Haddad Maia in Cincinnati on Saturday.
Alexandrova meantime will play compatriot Anna Kalinskaya next after she stunned American fifth seed Amanda Anisimova 7-5 6-4.
Two Aussie men are still alive in Cincy with Adam Walton and Alexei Popyrin both due to play their third round matches on Tuesday (local time).
Elsewhere in the women's draw, world No.1 Aryna Sabalenka kept her title defence alive by claiming a 7-6 (7-3) 4-6 7-6 (7-5) victory over Brit Emma Raducanu.
Fresh off their third-round clash at Wimbledon last month, Sabalenka and Raducanu, the 2021 US Open champion, produced another epic contest which lasted three hours nine minutes.
The Belarusian's two tiebreak wins takes her tally to 18 for the season, the most by any women's singles player in the professional era.
"Happy to get through this match. I really hope tomorrow I have a day off," said Sabalenka, who faces Spain's Jessica Bouzas Maneiro in the last 16.
Former champion Madison Keys booked her place in the fourth round, where she'll take on ninth seed Elena Rybakina, with a 6-4 6-0 win over Japan's Aoi Ito.
"Once I got a break early in the second (set) I wanted to run away with it and keep that momentum, and I did," sixth seed Keys said.
With Reuters.
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